The present invention is directed to a method and apparatus for predicting transient response characteristics of power supplies or other closed loop systems under arbitrary load conditions. The present invention is particularly directed to a method for predicting transient response characteristics for direct current, DC-to-DC, power supplies.
When designing certain systems, such as power supply, or power converter circuits, one must take into account the potential user's load characteristics. This consideration is especially important in the design of DC-DC converters because such converters are generally configured as a closed loop system that monitors its output, provides feedback indicating its output, and employs the feedback to adjust to maintain a constant DC output. In any feedback system, it is of significant importance that the feedback loop be stable. A simple example of an unstable feedback loop is the loud tone produced in the presence of audio feedback when a microphone is placed too close to a speaker producing signals originating at the microphone.
Today's electronic devices are more and more designed to be faster, smaller, and more reliable. This trend for product requirements is especially evident in portable electronic devices such as cellular telephones, electronic games, and portable computers. Some practical design consequences of this trend are that output voltages for DC-DC converters are getting lower and the stability of output of DC-DC converters is more difficult to attain for certain loads or applications.
The fact that a user's load characteristics figure so intimately in stability of DC-DC converter circuits, and the ever more stringent requirements for greater stability at lower voltages for modern electronic circuits have made present ways of predicting stability of a particular DC-DC converter circuit for a particular application uneconomical and not particularly reliable or accurate.
Nyquist developed criteria to assess the stability of a control loop (“regeneration Theory”, H. Nyquist, Bell System Technical Journal, January 1932). Bode (“Relations Between Attenuation and Phase in Feedback Amplifier Design”, Bell System Technical Journal, July 1940) expressed these criteria in terms of the phase (φ) and gain of a transfer function. According to this analysis, if gain (dB) and phase change (Δφ) of the loop gain are zero at the same frequency in a circuit, the circuit will be unstable.
As a practical engineering measure, one must design a circuit having ≧45° phase margin to reliably have a stable circuit. Phase margin is the value of phase when gain as a function of frequency crosses through zero from positive to negative. Thus, when gain is 0 dB, and gain is passing from positive to negative, phase must be ≧45° in order for the circuit under consideration to be stable with adequate margin.
Another measure of stability is to require that gain margin be ≧−7 to −10 dB. That is, when phase as a function of frequency crosses through zero, gain must be at least 7-10 dB in order that the circuit under consideration will be a stable circuit.
Presently, manufacturers of power supplies, and especially of DC-DC converters, use simulations, or laboratory measurements, or closed form analytical expressions, or all three of those methods for determining whether a particular circuit is stable with a particular load. Simulations are expensive in that they occupy large amounts of computer capacity and time. Closed form analytical expressions rely on simplifying assumptions that introduce significant errors. Laboratory measurements are an expensive approach to answering questions about a particular circuit-load stability in terms of human time and computer assets involved. Further, neither simulations, closed form analytical expressions nor laboratory experimentation are particularly accurate in predicting stability of converter apparatuses under various load conditions.
One result of ongoing efforts to predict stability with arbitrary loads is that manufacturers of power converters must essentially custom-tailor their products to user's loads on a case-by-case basis. Such a “job shop” approach to production precludes one's taking advantage of the economies of scale which could be enjoyed if a manufacturer could predict which loads were amenable to stable use with particular converters. That is, if manufacturers could predict stability for a particular converter circuit for a particular load without having to physically evaluate the converter circuit with the particular load, then the inefficiencies of customizing converter circuits for each discrete load criterion may be avoided.
A product designer is concerned with stability of the circuits that are incorporated in the products, but must also be concerned with the transient response characteristics of the circuits. That is, there must be consideration of the transient voltage characteristics and the settling time of the design. Settling time for a circuit or system is the time that lapses from a perturbation of the system until the parameter being measured (e.g. output voltage in the case of a typical power supply) is within a desired percentage of a desired value. Specifically, by way of example, settling time for a power supply may be the time for a power supply's output voltage to return to within 1-2% of a desired design output voltage for the power supply after the power supply is switched on. The transient voltage is the amplitude excursion of the output voltage during the settling time interval.
If manufacturers could also predict transient response characteristics for a particular converter circuit for a particular load without having to actually physically evaluate the converter circuit with the particular load, they would enjoy an added advantage in predictability of critical operational characteristics of their products.
Moreover, one may use the present invention to ascertain design characteristics necessary for products to exhibit certain operational characteristics. For example, one may determine that a product desirably should operate with a particular transient peak voltage and settling time for a particular current step-change. Using the present invention, a designer may determine appropriate design parameters for the desired product operating characteristics.
Manufacturers enjoying such an advantage in predictability of operational characteristics of their products vis-à-vis loads may produce converter apparatuses for “off-the-shelf” availability to customers with evaluation tools enabling customers to select which of the converters will accommodate the particular loads they are designing.
There is a need for a method for predicting transient response characteristics of power converters under arbitrary load conditions. This need is particularly acute in predicting transient response characteristics of DC-DC power converter circuits.
It would be particularly useful if transient response characteristics of power supply apparatuses could be predicted without having to test the power supply apparatus under the particular load condition for which a transient response determination is desired.
The method of the present invention allows evaluation of the transient response of a power supply apparatus for various load conditions without having to recharacterize the apparatus for each given load.